Faults during operation of permanent magnet electrical motors are fortunately very rare. However, in the situation that a fault does occur, since there is a permanent magnet rotor, the motor can in this case operate as a generator. Consequently, if rotor speed is sufficiently high a generated overvoltage will result causing electrical stress on components of any drive system connected to the motor. This can result in costly repairs and significant system down times.
One way to reduce any ill-effects of faults, is to over dimension any components that would be subjected to high voltage during faults. However, such over dimensioning is expensive and may also require more space to house physically larger components.
JP-2000 184 770 presents a solution where breakers are used to cut off an electrical connection between inverter and a permanent magnet AC motor. However, breakers are relatively slow and a faster solution would reduce any ill-effects during a fault.